Lng production system equipped with recondenser

ABSTRACT

An LNG production system including a boil off gas recondenser that can recondense boil off gas without using a BOG compressor and without depending on an LNG liquefaction process is provided.

TECHNICAL FIELD

The present invention relates to an LNG production system equipped witha recondenser (Recondenser) that recondenses (reliquefies) boil off gas(BOG, Boil off Gas).

BACKGROUND ART

FIG. 6 illustrates a general LNG production system. Natural gas (NG,Natural Gas) is fed to a CO₂ removal process 62 in a subsequent stage bya compressor 61. In the CO₂ removal process 62, CO₂ is removed from thenatural gas by using a predetermined solvent. The NG after removal fromwhich CO₂ is removed is fed to a dry process 63. In the dry process 63,predetermined drying treatment is applied to the NG after removal. Thedried NG is fed to a liquefaction process 64. In the liquefactionprocess 64, the dried NG is liquefied by using a liquid refrigerant thatis fed from a refrigeration system 65. The liquid natural gas (LNG)obtained by liquefaction is fed to an LNG storage tank 66. Subsequently,in an LNG loading process 68, LNG is fed from the LNG storage tank 66 ata predetermined timing (for example, at the timing at which the LNG istransferred to a transport ship tank, or the like). Here, LNG in the LNGstorage tank 66 sometimes evaporates by natural heat input and generatesBOG. Further, when LNG is transferred to the transport ship tank fromthe LNG storage tank 66 in the LNG loading process 68, a large amount ofBOG is sometimes generated. Further, piping is cooled when LNG istransferred to the transport ship tank, so that BOG is sometimesgenerated.

Many LNG storage tanks 66 which are installed in the LNG productionbases have large capacities, and design pressure thereof is generallyset at a vicinity of the atmospheric pressure from the viewpoints oftechnology and cost. Consequently, it is necessary to discharge BOG fromthe LNG storage tank 66 even at the time of a slight pressure increase.Further, the pressure increase also occurs by a piston effect (alsoreferred to as “pushup effect”) accompanying supply of LNG to the LNGstorage tank 66 from the liquefaction process 64, and therefore supplyof LNG to the LNG storage tank 66 causes regular discharge of BOG.Further, a flash loss of decompression accompanying charge to the LNGstorage tank 66 from a liquefier of the liquefaction process 64 alsooccurs. Note that LNG loss by a flash loss sometimes occupiesapproximately 50% of the BOG generation amount.

However, it is undesirable to discharge BOG into the atmosphere in termsof environmental and economical aspects, so that BOG is conventionallyreturned to the dry process 63 by a compressor 67, and is supplied tothe liquefaction process 64 with the dried NG. Thereby, it is possibleto reliquefy BOG. Alternatively, BOG is sometimes used as the heatsource for regeneration of a drying material or the like in the dryprocess 63.

However, in the case of reliquefaction of the BOG described above, atotal amount of NG that is fed to the liquefaction process 64 from thedry process 63 includes the recycled BOG. Further, use as the fuel gasmeans that the total amount of LNG which is produced cannot betransferred to the transport ship tank.

Further, it is required to bring BOG into a high pressure state in orderto reliquefy the BOG, so that the compressor 67 is required at the timeof recycle that returns the BOG to the dry process 63. Consequently,consumption of a large amount of energy is required to compress the BOG.

Further, when BOG is returned to the dry process 63 as described above,BOG can be reliquefied only at the lime of LNG production, so that whenBOG has to be discharged from the LNG storage tank at the time ofproducing no LNG, BOG has to be discharged into the air. That is, thetiming for reliquefaction is limited, and there is no flexibility inreliquefaction process.

The LNG production system described in Patent Literature 1 presents nomethod of solution to the above described problem.

CITATION LIST Patent Literature

[Patent Literature 1] U. S. Patent No. 2011/0094261

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide an LNG productionsystem including a recondenser that can recondense BOG (boil off gas)without using a BOG compressor and without depending on an LNGliquefaction process.

Solution to Problem

A first LNG production system of the present invention includes

a liquefier that cools and liquefies natural gas by a refrigerant thatis fed from a refrigerator,

an LNG tank that stores liquid natural gas (LNG) liquefied in theliquefier,

a transfer line for transferring the liquid natural gas from the LNGtank,

an LNG carrier that is disposed in a subsequent stage of the transferline and is for transferring the liquid natural gas,

a recondenser that recondenses (reliquefies) boil off gas that isgenerated by heat being given to the liquid natural gas, by therefrigerant fed from the refrigerator, and

a return line that feeds liquid natural gas that is liquefied to the LNGtank from the recondenser.

A second LNG production system of the present invention includes

a liquefier that cools and liquefies natural gas by a refrigerant thatis fed from a first refrigerator,

an LNG tank that stores liquid natural gas liquefied in the liquefier,

a transfer line for transferring the liquid natural gas from the LNGtank,

an LNG carrier that is disposed in a subsequent stage of the transferline and is for transferring the liquid natural gas,

a recondenser that recondenses boil off gas (BOG) that is generated byheat being given to the liquid natural gas, by a refrigerant fed fromthe second refrigerator, and

a return line that feeds liquid natural gas that is liquefied to the LNGtank from the recondenser.

In the above described invention, the refrigerant that is fed from thefirst refrigerator, and the refrigerant that is fed from the secondrefrigerator may be the same refrigerants, or may be differentrefrigerants. For example, as the refrigerant from the firstrefrigerator, a mixture such as hydrocarbon is cited, and as therefrigerant from the second refrigerator, nitrogen or the like is cited.

A third LNG production system of the present invention includes

a liquefier that cools and liquefies natural gas by a refrigerant thatis fed from a refrigerator,

an LNG tank that stores liquid natural gas liquefied in the liquefier,

a transfer line for transferring the liquid natural gas from the LNGtank,

an LNG carrier that is disposed in a subsequent stage of the transferline and is for transferring the liquid natural gas,

a recondenser that switches to perform, alternately, a firstrecondensation processing of liquefying boil off gas that is generatedby heat being given to the liquid natural gas, by the refrigerant fedfrom the refrigerator, and a second recondensation processing ofliquefying boil off gas by the refrigerant fed from the refrigerator anda refrigerant fed from a refrigerant buffer to process more boil off gasthan the boil off gas at a time of the first recondensation processing,and

a return line that feeds liquid natural gas that is liquefied to the LNGtank from the recondenser.

In the above described invention, the refrigerant that is fed from therefrigerator, and the refrigerant that is fed from the refrigerantbuffer may be the same refrigerants, or may be different refrigerants.For example, as the refrigerant from the refrigerator, a mixture such ashydrocarbon is cited, and as the refrigerant from the refrigerantbuffer, nitrogen or the like is cited.

In the aforementioned first recondensation processing, the refrigerantmay be fed to the recondenser from the refrigerator, and in the secondrecondensation processing, the refrigerant from the refrigerant buffermay be fed to the recondenser in addition to that the refrigerant is fedto the recondenser from the refrigerator. In the case of switching tothe second recondensation processing from the first recondensationprocessing, the operation of the refrigerator may be stopped, or therefrigerator may be continuously operated without stopping.

The recondenser may have a switch control section that switches thefirst recondensation processing and the second recondensation processingto each other.

The switch control section may switch from the first recondensationprocessing to the second recondensation processing, in the case oftransferring BOG to the LNG carrier.

The switch control section may switch from the first recondensationprocessing to the second recondensation processing when a pressure valuemeasured by a pressure gauge disposed in the LNG tank or the feed linethat feeds the BOG to the recondenser becomes a predetermined value ormore.

According to the above described respective configurations, in the caseof processing BOG in an amount in a predetermined range (a flow rate perunit time) or of a pressure value in a predetermined range that is setin advance, the first recondensation processing (processing ofliquefying BOG with the refrigerant from the refrigerator) is executed,and in the case of processing BOG that exceeds the above describedamount in the predetermined range or pressure value in the predeterminedrange, the second recondensation processing (processing of liquefactionalso by the refrigerant fed from the refrigerant buffer while keepingliquefaction by the refrigerant fed from the refrigerator) can beexecuted, so that BOG can be recondensed without using a BOG compressorand without depending on the LNG liquefaction process.

In each of the above described first to third LNG production system,predetermined treatment may be applied to the aforementioned natural gasthat is fed to the liquefier in advance. For example, each of the LNGproduction systems may include a removing device that removespredetermined impurities from natural gas, and a dryer that dries thenatural gas that is treated by the removing device.

The transfer line may be provided with piping and a sluice valve.

The return line may be provided with piping, a pump for feeding LNG andan automatic on-off valve.

A feed line that feeds the BOG to the recondenser from the LNG tank maybe included. The feed line may be provided with any one or more ofpiping, an automatic on-off valve, a flow rate control valve and apressure regulating valve.

A pressure gauge that measures the pressure of the LNG tank may beprovided. When a pressure value of the pressure gauge reaches apredetermined value or more, valves of the feed line and the return linemay open, and BOG may be fed to the recondenser through the feed line.

The recondenser may be controlled so as to increase a cooling capabilityof the recondenser when the pressure value of the pressure gaugeinstalled in the feed line reaches the predetermined value or more. Forexample, control may be performed so as to increase the feeding amountof the refrigerant that is fed from the refrigerator (the first or thesecond refrigerator), for example.

The LNG carrier may be, for example, a loading station container, aloading pier, a loading station truck, and the like.

A recovery line for returning BOG that is present in the LNG carrier tothe LNG tank may be provided.

In the third LNG production system, the refrigerant stored in therefrigerant buffer may be supplied from the refrigerator or an externalrefrigerator.

The recondenser may have a piping through which the refrigerant fed fromthe refrigerator passes, and a piping through which the refrigerant fedfrom the refrigerant buffer passes as separate components, and thereturning refrigerants may be returned together to the refrigerator.

The recondenser may have a first heat exchanger to which the refrigerantfed from the refrigerator is introduced, and a second heat exchanger towhich the refrigerant fed from the refrigerant buffer is introduced.

In each of the above described first to third LNG production systems,the recondenser preferably has the following configuration.

The recondenser is designed to recondense (liquefy) the boil off gas bya refrigerant under a pressure lower than an operating pressure of theLNG tank.

According to the configuration, BOG can be recondensed under thepressure lower than the operating pressure of the LNG tank without usingthe conventional BOG compressor.

The recondenser may be internally provided with a heat exchanger intowhich the refrigerant is introduced, and the BOG may be introduced intothe heat exchanger, and is cooled by the refrigerant. Thereby, BOG canbe effectively liquefied in a mode of the heat exchanger.

A volume (an external capacity) of the heat exchanger may be smallerthan an internal volume (an internal space capacity) of the recondenser,and the heat exchanger may be disposed in the internal space of therecondenser.

Thereby, BOG can be effectively liquefied in the mode of the heatexchanger. The liquefied LNG accumulates on the bottom of therecondenser. The accumulating LNG can be fed to the LNG tank by a liquidfeed pump.

The pressure in the recondenser or in the heat exchanger may beregulated as follows.

(1) Before sending BOG, feed the refrigerant, and pre-cool the inside ofthe recondenser or the inside of the heat exchanger. After a lapse of apredetermined time period, or when the inside of the recondenser or theinside of the heat exchanger reaches a predetermined temperature, startintroduction of BOG.(2) The introduced BOG is liquefied, and accumulates on the bottom ofthe recondenser or the heat exchanger. The liquefied LNG accumulating onthe bottom can be fed to the LNG tank by the pump, a pressurizing deviceor the like.

A fourth LNG production system of the present invention includes

a liquefier that cools and liquefies natural gas by a refrigerant thatis fed from a refrigerator,

an LNG tank that stores liquid natural gas liquefied in the liquefier,

a transfer line for transferring the liquid natural gas from the LNGtank,

an LNG carrier that is disposed in a subsequent stage of the transferline and is for transferring the liquid natural gas,

an LNG lead-out line that leads out the liquid natural gas from the LNGtank,

a sub-cooler that is provided in the LNG lead-out line and cools theliquid natural gas with a refrigerant (for example, liquid nitrogen orthe like),

a recondenser that recondenses boil off gas that is generated by heatbeing given to the liquid natural gas, by the liquid natural gas cooledin the sub-cooler, and

a return line that feeds liquid natural gas that is liquefied to the LNGtank from the recondenser.

In the present invention, the recondenser may recondense (liquefy) boiloff gas with LNG that is cooled by the sub-cooler under a pressure lowerthan an operating pressure of the LNG tank.

According to the configuration, the liquid natural gas is firstly cooledby using the refrigerant such as LN₂, and boil off gas is liquefied withthe cooled liquid natural gas. Thereby, recondensation of boil off gascan be effectively performed under the pressure lower than the operatingpressure of the LNG tank.

In the fourth invention, the sub-cooler may be controlled so that liquidnatural gas has a higher temperature than a solidifying point of theliquid natural gas by the pressure regulating valve or the flow rateregulating valve that is installed in the refrigerant line in which therefrigerant flows.

In the fourth invention, two or more of the sub-coolers may be adopted.In the case of the two sub-coolers, the first recondensation processingof liquefying BOG by the refrigerant fed from the first sub-cooler, andthe second recondensation processing of liquefying BOG by therefrigerant fed from the first sub-cooler and the refrigerant fed fromthe second sub-cooler in order to process more boil off gas than theboil off gas at a time of the first recondensation processing may beperformed by switching the first recondensation processing and thesecond recondensation processing to each other. In the case of switchingfrom the first recondensation processing to the second recondensationprocessing, the operation of the refrigerator may be stopped, or therefrigerator may be continuously operated without stopping.

The recondenser may have a switch control section that switches thefirst recondensation processing and the second recondensation processingto each other.

The switch control section may switch from the first recondensationprocessing to the second recondensation processing in the case oftransferring boil off gas to the LNG carrier.

The switch control section may switch from the first recondensationprocessing to the second recondensation processing when a pressure valuemeasured by a pressure gauge disposed in the LNG tank or the feed linethat feeds the BOG to the recondenser becomes a predetermined value ormore.

The refrigerant in the second sub-cooler may be supplied from therefrigerant buffer in which the refrigerant is stored in advance.

In the above described LNG production system, the pump for feedingliquid natural gas (LNG) to the transfer line from the LNG tank may bean in-tank type pump that is installed inside the LNG tank, or may be apump that is disposed on the transfer line.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration example of an LNGproduction system of embodiment 1.

FIG. 2 is a diagram illustrating a configuration example of an LNGproduction system of embodiment 2.

FIG. 3 is a diagram illustrating a configuration example of an LNGproduction system of embodiment 3.

FIG. 4A is a diagram illustrating a configuration example of arecondenser.

FIG. 4B is a diagram illustrating a configuration example of therecondenser.

FIG. 4C is a diagram illustrating a configuration example of therecondenser.

FIG. 5A is a diagram illustrating a configuration example of an LNGproduction system in embodiment 4.

FIG. 5B is a diagram illustrating a configuration example of therecondenser.

FIG. 5C is a diagram illustrating a configuration example of therecondenser.

FIG. 6 is a diagram illustrating a configuration example of aconventional LNG production system.

DESCRIPTION OF EMBODIMENTS

Hereunder, several embodiments of the present invention will bedescribed. The embodiments described as follows explain only examples ofthe present invention. The present invention is not limited by thefollowing embodiments in any way, and also includes various modifiedmodes that are carried out within the range without changing the gist ofthe present invention. Note that all of components described as followsare not always indispensable components of the present invention.

Embodiment 1

An LNG production system 1 of embodiment 1 will be described withreference to FIG. 1. The LNG production system 1 has a first line L1 fortransferring natural gas to a process in a subsequent stage, acompressor 11 and a second line L2 (a pipe, for example). As the processin the subsequent stage, a removing unit 12 is disposed, and apredetermined substance (CO2, for example) is removed from NG here.Next, the NG after removal is fed to a dryer 13 through a third line L3,and is subjected to drying treatment. Next, the dried NG is fed to aliquefier 14 through a fourth line L4 and is liquefied. A refrigerant (aliquid refrigerant) is fed to the liquefier 14 from a refrigerator 15 tocool NG, and LNG is obtained. Further, the refrigerant which issubjected to heat exchange returns to the refrigerator 15 in anevaporated state. LNG is fed to the LNG tank 16 through a fifth line L5and stored. The first line L1 to the fifth line L5 are configured bypipes and on-off valves, for example. A predetermined control device(controller) controls operation of the respective devices, opening andclosing of the valves, a production amount of LNG, and the like of theLNG production system 1.

In the LNG tank 16, an in-tank type first pump P1 is disposed, and LNGin the tank is fed into an LNG carrier 18 through a transfer line L6 bythe first pump P1. As the LNG carrier 18, for example, a loading stationcontainer, a loading pier, a loading station track, and the like arecited. BOG that is present in the LNG carrier 18 is fed to the LNG tank16 through a recovery line A2. Instead of or in addition to the recoveryline A2, a second feed line for feeding BOG present in the LNG carrier18 to a recondenser 17 may be provided.

In the LNG tank 16, BOG is generated by heat input. Further, when LNG isfed from the liquefier 14, BOG is also generated. Further, when LNG isfed to the LNG carrier 18, BOG is also generated. In this way, BOG inthe LNG tank 16 is fed to the recondenser 17 through a first feed lineA1. Further, BOG in the transfer line L6 is fed to the recondenser 17through a third feed line A3.

A refrigerant (liquid refrigerant) is introduced into the recondenser 17through a refrigerant line B1 from the refrigerator 15. By therefrigerant, BOG fed by each of the feed lines is recondensed(liquefied). A configuration of the recondenser 17 is described later.The LNG that is obtained by being recondensed (liquefied) is returned tothe LNG tank 16 through a return line A4. In the return line A4, asecond pump P2 is disposed, and LNG is fed to the LNG tank 16 byoperating the second pump P2.

According to the present embodiment, a series of processes is notrequired, which feeds BOG to the dryer and feeds BOG to the liquefierwith NG to liquefy BOG as in the conventional art. Consequently, it isnot necessary to operate the entire LNG production system, and only therefrigerator 15 can be operated. The recondenser 17 can recondense BOGto LNG, so that all of the liquefaction capability of the liquefier 14can be used in liquefaction of NG that is fed from the dryer.

(Recondenser)

FIGS. 4A and 4B illustrate an embodiment of the recondenser 17. In FIG.4A, the recondenser 17 has an outer wall 171, and a heat exchanger 172covered with the outer wall 171. A refrigerant (a liquid refrigerant) isintroduced into the heat exchanger 172 from the refrigerator 15 througha refrigerant line B1, and BOG is cooled by cold energy of therefrigerant. The refrigerant evaporates and returns to the refrigerator15 through a refrigerant return line B2. LNG is fed to the LNG tank 16from the recondenser 17 by the second pump P2.

The recondenser 17 is designed to recondense (liquefy) BOG with therefrigerant under a pressure lower than an operating pressure of the LNGtank 16.

The first feed line A1 may be provided with a safety valve for a timewhen the pressure in the LNG tank 16 becomes abnormally high. Further,in the first feed line A1, an automatic on-off valve 42 for performingfeeding control of BOG to the recondenser 17 is provided. Further, inthe first feed line A1, a pressure gauge, and a pressure regulatingvalve 41 controlled in accordance with a value of the pressure gauge areprovided.

The operating pressure in the LNG tank 16 is an average of 1.2 barA (120KPaA) in absolute pressure, and is controlled within ±15% as upper andlower limit values. When a large amount of BOG is generated, a tankinternal pressure becomes high. The tank internal pressure is measuredwith a pressure gauge, and based on a measurement result (a conversionresult), a valve control section (not illustrated) controls opening andclosing of the automatic on-off valve 42. For example, when the tankinternal pressure becomes 1.3 times as high as 1.2 barA (120 KPaA), BOGis fed to the recondenser 17. As for the pressure regulating valve 41,in-pipe pressure of the first feed line A1 is measured, and a valveopening degree is controlled based on the measurement result.

The refrigerant which is supplied from the refrigerator 15 may be anymedium having a lower temperature than a boiling point of LNG, and LN2may be used, for example.

Internal pressure of the heat exchanger 172 is controlled to be apressure lower than the operating pressure (average of 1.2 barA (120KPaA) in absolute pressure) of the LNG tank 16, in a BOG recondensationprocessing. The internal pressure of the heat exchanger 172 is measuredwith a pressure gauge, and is regulated to be lower than the operatingpressure of the LNG tank 16. In the present embodiment, the refrigerantcontacts BOG in the heat exchanger 172, whereby a volume of BOGdecreases by liquefaction and the pressure in the heat exchanger 172 isreduced. During operation, the low pressure state is kept by therefrigerant being continuously supplied. The internal pressure of theheat exchanger 172 is regulated by controlling a flow rate of therefrigerant. A flow rate regulating valve (not illustrated) is providedin the refrigerant feed line B1, and the flow rate of the refrigerantmay be controlled with the flow rate regulating valve in accordance witha measurement result of the pressure gauge that measures the internalpressure of the above described heat exchanger 172.

Note that the recondenser 17 is not limited to a mode of the heatexchanger 172, but may be a mode in which BOG and the refrigerant arebrought into direct contact with each other. As a method for contactingboth BOG and the refrigerant to each other, means of spraying therefrigerant by a shower, means of contacting both of them by using afiller and the like are cited. A lower portion of the heat exchanger 172and the return line A4 are connected. An automatic on-off valve (notillustrated) provided in the return line A4 is controlled to open andclose, and the second pump P2 is controlled, whereby LNG can be fed backto the LNG tank 16 from the recondenser 17. A processing procedure ofrecondensation processing of boil off gas (BOG) will be describedhereunder.

(1) Feed the refrigerant to the heat exchanger 172 from the refrigerator15, and pre-cool the heat exchanger 172, when the tank internal pressureof the LNG tank 16 exceeds a first threshold value. A temperature of therefrigerant is preferably set at a temperature higher than an LNGsolidifying point, and lower than a temperature of LNG in the LNG tank16, for example. The temperature of LNG which is cooled may be set basedon an amount of BOG and an amount of LNG which is cooled.(2) When the heat exchanger 172 reaches a predetermined temperature orless, regulate the supply amount of the refrigerant by the flow rateregulating valve (not illustrated) provided in the refrigerant line B1to keep the temperature.(3) When the tank internal pressure of the LNG tank 16 exceeds a secondthreshold value (the second threshold value>the first threshold value),open the automatic on-off valve 42 and the pressure regulating valve 41,and introduce BOG directly into the heat exchanger 172 of therecondenser 17 from the LNG tank 16. At the time of introducing the BOG,regulate the supply amount of the refrigerant to the heat exchanger 172,and keep the inside of the heat exchanger 172 under a negative pressure(or under a pressure lower than the operating pressure of the LNG tank16).(4) The heat exchanger 172 is pre-cooled, and BOG is cooled immediatelyand changes state to LNG, and the LNG drops onto a bottom of the heatexchanger 172.(5) LNG is fed back to the LNG tank 16 through the return line A4.(6) Close the respective valves after the recondensation processingends.

It is assumed that a condition of (3) (the tank internal pressure>thesecond threshold value) is established during processing of (1) and/or(2), so that BOG may be configured to be discharged from the LNG tank 16with a safety valve (not illustrated), or BOG may be discharged toexternal air by a vent not illustrated.

The above described “first threshold value” is a pressure that is 1.26times as high as 1.2 barA (120 KPaA), for example.

The above described “second threshold value” is a pressure that is 1.3times as high as 1.2 barA (120 KPaA), for example.

A pressure regulating valve (not illustrated) or a flow rate regulatingvalve (not illustrated) may be installed in the refrigerant line B1, anda refrigerant feed amount (VN) and a BOG feed amount (VB) may becontrolled to VN>VB.

Other Embodiments

A recondenser in FIG. 4B will be described. In FIG. 4B, in therecondenser 17, a volume (an external capacity) of the heat exchanger172 is smaller than an internal volume (an internal space capacity) ofthe recondenser 17, and the heat exchanger 172 is disposed in aninternal space 173 of the recondenser. In the mode of the heatexchanger, BOG can be effectively liquefied. The liquefied LNGaccumulates on a bottom of the internal space 173 of the recondenser 17.The accumulating LNG can be fed to the LNG tank 16 by the second pumpP2.

An upper portion (preferably an upper side of the heat exchanger 172) ofthe internal space 173 of the recondenser 17 and the first feed line A1are directly connected. Further, a lower portion of the internal space173 of the recondenser 17 and the return line A4 are directly connected.

An internal pressure of the recondenser 17 is controlled to be apressure lower than the operating pressure (an average of 1.2 barA (120KPaA) in absolute pressure) of the LNG tank 16 in the BOG recondensationprocessing. The internal pressure of the recondenser 17 is measured by apressure gauge, and is regulated to be lower than the operating pressureof the LNG tank 16.

In the present embodiment, the refrigerant is fed to the heat exchanger172, and thereby the inside of the recondenser 17 is cooled. When BOG isintroduced into the cooled recondenser 17, the volume of BOG decreasesby liquefaction and the pressure inside the recondenser 17 is reduced.During operation, the refrigerant is continuously supplied to the heatexchanger 172 and thereby continues to cool the inside of therecondenser 17 to liquefy BOG to keep the inside of the recondenser 17in a low-pressure state. The internal pressure of the recondenser 17 isregulated by controlling the flow rate of the refrigerant. A flow rateregulating valve is provided in the refrigerant line B1, and the flowrate of the refrigerant may be controlled by the flow rate regulatingvalve in accordance with the measurement result of the pressure gaugethat measures the internal pressure of the above described recondenser17, the flow rate of the refrigerant may be controlled by controllingthe opening degree of the automatic on-off valve provided in therefrigerant line B1, or both of them may be controlled.

A processing procedure of the recondensation processing of boil off gas(BOG) will be described hereunder.

(1) Feed the refrigerant to the heat exchanger 172, and pre-cool theheat recondenser 17, when the tank internal pressure of the LNG tank 16exceeds a first threshold value. A temperature of the refrigerant ispreferably set at a temperature higher than the LNG solidifying point,and lower than a temperature of LNG in the LNG tank 16, for example. Thetemperature of LNG which is cooled may be set based on an amount of BOGand an amount of LNG which is cooled.(2) When the recondenser 17 reaches a predetermined temperature or less,regulate the supply amount of the refrigerant by the flow rateregulating valve (not illustrated) provided in the refrigerant line B1to keep the temperature.(3) When the tank internal pressure of the LNG tank 16 exceeds a secondthreshold value (the second threshold value>the first threshold value),open the automatic on-off valve 42 and the pressure regulating valve 41,and introduce BOG into the recondenser 17 from the LNG tank 16. At thetime of introducing the BOG, regulate the supply amount of therefrigerant to the heat exchanger 172, and keep the inside of the heatexchanger 172 under a negative pressure (or under a pressure lower thanthe operating pressure of the LNG tank 16).(4) The recondenser 17 is pre-cooled, and BOG is cooled immediately andchanges state to LNG, and the LNG accumulates on the bottom of therecondenser 17.(5) LNG that accumulates on the bottom of the recondenser 17 is fed backto the LNG tank 16 through the return line A4.(6) Close the respective valves after the recondensation processingends.

It is assumed that a condition of (3) (the tank internal pressure>thesecond threshold value) is established during processing of (1) and/or(2), so that BOG may be configured to be discharged from the LNG tank 16with a safety valve (not illustrated), or BOG may be discharged toexternal air by a vent not illustrated.

The above described “first threshold value” is a pressure that is 1.26times as high as 1.2 barA (120 KPaA), for example.

The above described “second threshold value” is a pressure that is 1.3times as high as 1.2 barA (120 KPaA), for example.

A pressure regulating valve (not illustrated) or a flow rate regulatingvalve (not illustrated) may be installed in the refrigerant line B1, anda refrigerant feed amount (VN) and a BOG feed amount (VB) may becontrolled to VN>VB.

Embodiment 2

An LNG production system 2 of embodiment 2 will be described with use ofFIG. 2. Components with the same reference signs as those in the LNGproduction system 1 of embodiment 1 have the same functions, andtherefore, explanation of the components will be omitted, or will bemade briefly.

The LNG production system 2 of embodiment 2 has the first refrigerator15 and a second refrigerator 20. The first refrigerator feeds arefrigerant to a cooling device 14. The second refrigerator 20 feeds therefrigerant (liquid refrigerant) to the recondenser 17 through arefrigerant line C1 (corresponding to B1 in FIG. 1), and returns therefrigerant used as a cold source in the recondenser 17 through a returnline C2 (corresponding to B2 in FIG. 1).

Consequently, since the second refrigerator 20 is provided separatelyfrom the first refrigerator 15, it is not necessary to supply therefrigerant to the operating cooling device 14 from the largerefrigerator and supply the refrigerant to the recondenser 17, it is notnecessary to install the refrigerator that is larger than necessary, andonly a medium or small refrigerator can be installed, so that theinstallation space can be small, and the initial cost and the runningcost can be reduced.

Embodiment 3

An LNG production system 3 of embodiment 3 will be described withreference to FIG. 3. Components with the same reference signs as thosein the LNG production system 1 of embodiment 1 have the same functions,and therefore, explanation of the components will be omitted, or will bemade briefly.

The recondenser 17 of the LNG production system 3 of embodiment 3 canperform a first recondensation processing of liquefying BOG by therefrigerant fed from the refrigerator 15, and a second recondensationprocessing that liquefies BOG by the refrigerant fed from therefrigerator 15 and a refrigerant fed from a refrigerant buffer 30 inorder to process more BOG than BOG at the time of the firstrecondensation processing by switching the first and the secondrecondensation processings to each other.

In the refrigerant buffer 30, the refrigerant is supplied from therefrigerator 15 through a first supply line E1 and/or the refrigerant issupplied from an external refrigerant source through a second supplyline E2 and is stored in advance. When the recondenser 17 is operated,the refrigerant is introduced into the recondenser 17 through a bufferline D1 from the refrigerant buffer 30.

The recondenser 17 has a switch control section (not illustrated) thatswitches the first recondensation processing and the secondrecondensation processing to each other.

When BOG is transferred to the LNG carrier 18, the switch controlsection can switch from the first recondensation processing to thesecond recondensation processing in response to a timing of start oftransfer that is scheduled, or a timing at which a detecting sectiondetects that LNG is transferred from the LNG tank 16, for example. Asthe detecting section, there are cited a detecting section that detectsthat the transportation ship enters a harbor, a detecting section thatdetects that the automatic on-off valve of the transfer line L6 opens, adetecting section that uses a control signal for controlling theautomatic on-off valve as a detection signal, a detecting section thatdetects that a measurement result of a flow meter disposed in thetransfer line L6 reaches a threshold value or more and the like.

Further, the switch control section can switch from the firstrecondensation processing to the second recondensation processing when apressure value of the inside of the LNG tank 16 measured by the pressuregauge, or a pressure value measured by a pressure gauge disposed in atleast any one of the feed line A1, the recovery line A2 and the feedline A3 reaches a predetermined value or more.

An example of the recondenser 17 of embodiment 3 will be described withFIG. 4C. The recondenser 17 has a first heat exchanger 172 and a secondheat exchanger 174 in an internal space 173 thereof. A refrigerant isintroduced into the first heat exchanger 172 from the refrigerator 15 ata time of the first recondensation processing, and cools BOG. When ageneration amount of BOG is large, the switch control section switchesfrom the first recondensation processing to the second recondensationprocessing. While the first heat exchanger 172 is operated, the secondheat exchanger 174 is further operated. The refrigerant is introducedinto the second heat exchanger 174 through the buffer line D1 from therefrigerant buffer 30. Thereby, cooling by the two heat exchangers isexecuted, so that at a peak time (for example, in a case of processing alarge amount of BOG that is generated when LNG is fed into the LNGtransport ship, for example) at which the amount of BOG that is fed islarger than the amount of BOG at a normal time, BOG is also effectivelycooled to be converted into LNG and the LNG can be returned to the LNGtank 16. Note that the refrigerant that is used in the second heatexchanger 174 is configured to join the refrigerant return line B2 ofthe first heat exchanger 172 through a refrigerant return line D2, butthe present invention is not limited to this, and the refrigerant returnline D2 may be connected to the refrigerator 15.

Further, when BOG is fed to the LNG carrier 18, the switch controlsection can switch from the second recondensation processing to thefirst recondensation processing in response to a timing of end oftransfer that is scheduled, or timing at which a detecting sectiondetects that LNG transfer from the LNG tank 16 is completed, forexample. As the detecting section, there are cited a detecting sectionthat detects that the automatic on-off valve in the transfer line L6 isclosed, a detecting section that detects the control signal thatcontrols the automatic on-off valve as a detection signal, a detectingsection that detects that the measurement result of the flow meterdisposed in the transfer line L6 becomes a threshold value or less andthe like.

Further, the switch control section can switch from the secondrecondensation processing to the first recondensation processing when apressure value of the inside of the LNG tank 16 measured by the pressuregauge, or a pressure value measured by a pressure gauge disposed in atleast one of the feed line A1, the recovery line A2 and the feed line A3becomes less than a predetermined value.

Other Embodiments

In the above described embodiments, two heat exchangers are disposed inthe recondenser 17, and cooling capabilities of the heat exchangers maybe the same or different.

In the present embodiment, a combination of the refrigerant buffer andthe heat exchanger is provided to be one, but the present invention isnot limited to this, and two or more combinations may be provided.

Embodiment 4

An LNG production system 5 in embodiment 4 will be described withreference to FIGS. 5A to 5B. Components with the same reference signs asthose in the LNG production system 1 of embodiment 1 have the samefunctions, and therefore, explanation of the components will be omitted,or will be made briefly.

In embodiment 4, the refrigerator in the LNG production system is notused, but LNG in the LNG tank is used. That is, in embodiment 4, LNG issub-cooled to a predetermined temperature by a refrigerant, and LNG isfed to the recondenser to be brought into contact with BOG to liquefyBOG.

Embodiment 4 has a first feed line A1 that feeds BOG from the LNG tank16, an LNG lead-out line E1 that leads out LNG from the LNG tank 16, asub-cooler 52 that cools LNG with the refrigerant, a recondenser 57 thatliquefies BOG that is fed through the first feed line A1 for BOG by LNGthat is cooled in the sub-cooler 52, under a pressure lower than theoperating pressure of the LNG tank 16, and a return line A4 that returnsLNG that is BOG liquefied in the recondenser 57 to the LNG tank 16. Therespective components will be described in detail hereunder.

The first feed line A1 may be provided with a safety valve (notillustrated) for a time when the pressure in the LNG tank 16 becomesabnormally high. Further, the first feed line A1 is provided with anautomatic on-off valve 42 and a pressure regulating valve 41 forperforming feeding control of BOG to the condenser 10.

The operating pressure in the LNG tank 16 is an average of 1.2 barA (120KPaA) in absolute pressure, and is controlled within ±15% as an upperand lower limit values. When a large amount of BOG is generated, thetank internal pressure becomes high. The tank internal pressure ismeasured by a pressure gauge, and a valve control section (notillustrated) controls opening and closing of the automatic on-off valve42 based on a measurement result (conversion result) thereof. Forexample, when the tank internal pressure becomes 1.3 times as high as1.2 barA (120 KPaA), BOG is fed to the recondenser 57. The pressureregulating valve 41 measures a pipe internal pressure of the first feedline A1, and controls a valve opening degree based on a measurementresult.

LNG is introduced into the sub-cooler 52 through the LNG lead-out lineE1 from the LNG tank 16. The valve control section (not illustrated)performs opening and closing control of the automatic on-off valve 51provided in the LNG lead-out line E1, and controls a liquid feed pumpP5, whereby LNG is fed to the sub-cooler 52 from the LNG tank 16, and isfed to the recondenser 57 in a subsequent stage. The in-tank liquid feedpump P1 may be configured to feed LNG, instead of the liquid feed pumpP5.

The refrigerant in the sub-cooler 52 can be a medium having atemperature lower than the boiling point of LNG, and LN2 is used in thepresent embodiment. LN2 is introduced into the sub-cooler 52 through arefrigerant line F1 from an LN2 source (LN2 tank, for example), and isused as a cold source for cooling LNG passing through an inside of thesub-cooler 52. LN2 may be gasified, or may be discharged as a fluid inwhich a liquid and gas are mixed, when LN2 is discharged from thesub-cooler 52 through a discharge line F2. The fluid (LN2 and/or GN2)which is discharged may be subjected to discharge treatment to theatmosphere or may be subjected to recycle processing.

In the sub-cooler 52, LNG may be controlled to have a temperature higherthan the LNG solidifying point by a pressure regulating valve (notillustrated), or a flow rate regulating valve (not illustrated) that isinstalled in the refrigerant line F1 in which the refrigerant (LN2)flows.

An internal pressure of the recondenser 57 is controlled to be apressure lower than the operating pressure (an average of 1.2 barA (120KPaA) in the absolute pressure) of the LNG tank 16 in the BOGrecondensation processing. The internal pressure of the recondenser 57is measured by the pressure gauge, and is regulated to be lower than theoperating pressure of the LNG tank 16.

In the present embodiment, LNG that is cooled in the sub-cooler 52contacts BOG in the recondenser 57, whereby the volume of BOG decreasesby liquefaction, and the pressure in the recondenser 57 is reduced.During operation, the low pressure state is kept by the cooled LNG beingcontinuously supplied. The internal pressure of the recondenser 57 isregulated by controlling the flow rate of the cooled LNG. A flow rateregulating valve is provided in an LNG feed line E2 between thesub-cooler 52 and the recondenser 57, and the flow rate of LNG may becontrolled by the flow rate regulating valve in accordance with ameasurement result of the pressure gauge that measures the internalpressure of the above described recondenser 57, the flow rate of LNG maybe controlled by controlling the opening degree of an automatic on-offvalve 51, or both of them may be controlled.

The BOG that is introduced into the recondenser 57 is brought intocontact with cooled LNG, whereby BOG is liquefied to be LNG, and LNGaccumulates on a bottom of the recondenser 57. As a method for bringingboth BOG and LNG into contact with each other, there are means ofspraying LNG cooled in the sub-cooler 52 by shower, means of bringingboth of them into contact with each other by using a filler, and thelike.

A lower part of the recondenser 57 and the return line A4 are connected.The valve control section (not illustrated) performs on-off control ofan automatic on-off valve 54 provided in the return line A4, andcontrols the liquid feed pump P2, whereby the valve control section canfeed LNG back to the LNG tank 16 from the recondenser 57.

A processing procedure of the recondensation processing of BOG will bedescribed hereunder. The respective valves 41 to 42, 51 and 54 are inclosed states except for the recondensation processing.

(1) Feed the refrigerant (LN2, for example) to the sub-cooler 52, whenthe tank internal pressure of the LNG tank 16 exceeds a first thresholdvalue.(2) When the sub-cooler 52 reaches a predetermined temperature or less,feed LNG to the sub-cooler 52 from the LNG tank 16, and is cooled. Forexample, a temperature of the LNG which is cooled is preferably set at atemperature higher than the LNG solidifying point, and lower than thetemperature of LNG in the LNG tank 16. The temperature of LNG which iscooled may be set based on the amount of BOG and the amount of LNG whichis cooled.(3) Feed the cooled LNG to the recondenser 57, and pre-cool therecondenser 57. The automatic on-off valve 54 of the return line A4 isclosed.(4) When the tank internal pressure of the LNG tank 16 exceeds a secondthreshold value (the second threshold value>the first threshold value),open the automatic on-off valve 42 and the pressure regulating valve 41,and introduce BOG to the recondenser 57 from the LNG tank 16.(5) The recondenser 57 is pre-cooled, and the cooled LNG is introducedinto the recondenser 57 with BOG, whereby BOG is cooled and changesstate to LNG, and the LNG accumulates on the bottom of the recondenser57.(6) When LNG that accumulates on the bottom of the recondenser 57reaches a predetermined amount (or at a predetermined timing), open theautomatic on-off valve 54, control the liquid feed pump P2, and feed LNGto the LNG tank 16 from the recondenser 57.(7) Close the respective valves after the recondensation processingends.

It is also assumed that a condition of (4) (the tank internalpressure>the second threshold value) is established during processing of(1) to (3), so that BOG may be configured to be discharged from the LNGtank 16 with a safety valve (not illustrated), or BOG may be dischargedto external air by a vent not illustrated.

The above described “first threshold value” is the pressure that is 1.26times as high as 1.2 barA (120 KPaA), for example.

The above described “second threshold value” is the pressure that is 1.3times as high as 1.2 barA (120 KPaA), for example.

Embodiment 5

An LNG production system of embodiment 5 will be described with use ofFIG. 5C. Components with the same reference signs as those in the LNGproduction systems 1 and 5 of embodiments 1 and 4 have the samefunctions, and therefore, explanation of the components will be omitted,or will be made briefly.

In embodiment 5, a first and second sub-coolers are disposed, and afirst recondensation processing of liquefying BOG by a refrigerant fedfrom the first sub-cooler 52, and a second recondensation processing ofliquefying BOG by the refrigerant fed from the first sub-cooler 52 and arefrigerant fed from a second sub-cooler 521 in order to process moreBOG than BOG at the time of the first recondensation processing areswitched. According to the embodiment, in the case of processing BOG inan amount in a predetermined range (a flow rate per unit time) or of apressure value in a predetermined range set in advance, the firstrecondensation processing (processing of liquefying BOG with LNG cooledin the first sub-cooler) is executed, and in the case of processing BOGexceeding the amount in the predetermined range or of the pressure valuein the predetermined range described above, the second recondensationprocessing (simultaneously performing the processing of liquefying BOGby LNG cooled in the second sub-cooler while keeping the processing ofliquefying BOG by LNG cooled in the first sub-cooler) can be executed.

The switch control section (not illustrated) may switch from the firstrecondensation processing to the second recondensation processing in thecase of transferring BOG to the LNG carrier, or may switch from thefirst recondensation processing to the second recondensation processingwhen the pressure value that is measured by the pressure gauge disposedin the LNG tank or in the feed line A1 that feeds BOG to the recondenser57 reaches a predetermined value or more.

In the present embodiment, the refrigerant which is fed to the firstsub-cooler 52, and the refrigerant which is fed to the second sub-cooler521 may be the same refrigerants, or may be different refrigerants. Forexample, as the refrigerant to the first sub-cooler 52, a mixture suchas hydrocarbon can be cited, and as the refrigerant to the secondsub-cooler 521, nitrogen and the like are cited.

The switch control section can perform switching from the firstrecondensation processing to the second recondensation processing in thetiming of embodiment 3 described above. When the processing is switchedto the second recondensation processing, the valve control section (notillustrated) performs opening and closing control of a sluice valve 53,feeds LNG to the second sub-cooler 521, and feeds LNG to the recondenser57 in the subsequent stage. That is, in the first recondensationprocessing, cooled LNG is fed to the recondenser 57 through the LNG feedline E2 from the first sub-cooler 52, but the processing is switched tothe second recondensation processing, and cooled LNG is fed to therecondenser 57 through an LNG feed line E21 from the second sub-cooler521, in addition to the cooled LNG being fed to the recondenser 57 fromthe first sub-cooler 52.

The refrigerant of the second sub-cooler 521 can be any medium with atemperature lower than the boiling point of LNG, and LN2 is used in thepresent embodiment. LN2 is introduced into the second sub-cooler 521through a refrigerant line F11 from an LN2 source (an LN2 tank, forexample), and is used as a cold source for cooling LNG that passesthrough an inside of the second sub-cooler 521. LN2 may be gasified, ormay be discharged as a fluid in which a liquid and gas are mixed, whenthe LN2 is discharged through a discharge line F21 from the secondsub-cooler 521. The fluid (LN2 and/or GN2) which is discharged may besubjected to discharge treatment into the atmosphere, or may besubjected to recycling processing. Further, in the second sub-cooler521, LNG may be controlled to have a temperature higher than the LNGsolidifying point, by a pressure regulating valve (not illustrated) or aflow rate regulating valve (not illustrated) that is installed in arefrigerant line F11 in which the refrigerant (LN2) flows.

Other Embodiments

In embodiments 4 and 5 described above, the respective automatic on-offvalves, pressure regulating valves and liquid feed pumps are provided inthe respective lines, but some or all of them may be omitted inaccordance with the use purpose without being limited to the abovedescribed dispositions.

Further, in embodiments 4 and 5 described above, in the BOGrecondensation processing, LNG that is cooled is fed to pre-cool therecondenser 57 before BOG is fed into the recondenser 57, but thepresent invention is not limited to this, and cooled LNG and BOG may befed together. In that case, a feeding amount (VL) of the cooled LNG anda feeding amount (VB) of BOG may be controlled to VL>VB. The flow rateregulating valves may be provided in the respective LNG introductionline E1, line E2 and first feed line A1 for BOG to perform flow ratecontrol of the respective feeding amounts.

Further, in embodiments 4 and 5 described above, the liquid feed pump P2is provided in the return line A4, but a configuration in which theliquid feed pump is not provided in the return line A4 may be adopted.LNG changed in state from BOG in the recondenser 57 may be fed into theLNG tank 16 by the gravity.

Further, the refrigerant in the aforementioned second sub-cooler 521 maybe supplied from a refrigerant buffer in which the refrigerant is storedin advance.

In each of all the embodiments described above, the pump P1 is of anin-tank type, but the pump P1 is not limited to this, and the pump P1may be a pump disposed on the transfer line L6.

REFERENCE SIGNS LIST

1 LNG production system

14 Cooling device

15 Refrigerator

16 LNG tank

17 Recondenser

18 LNG carrier

L6 Transfer line

1-12. (canceled)
 13. An LNG production system, comprising: a liquefierthat cools and liquefies natural gas by indirect heat exchange with arefrigerant that is fed from a first refrigerator; an LNG tank thatstores liquid natural gas liquefied in the liquefier; a transfer linefor transferring the liquid natural gas from the LNG tank to an LNGcarrier; a recondenser that recondenses boil off gas that is generatedby the liquid natural gas, the recondensing performed by indirect heatexchange with the refrigerant fed from a second refrigerator; and areturn line that feeds the recondensed boil off gas to the LNG tank fromthe recondenser.
 14. The LNG production system of claim 13, wherein thefirst refrigerator and the second refrigerator are the same.
 15. The LNGproduction system of claim 13, wherein the first refrigerator and thesecond refrigerator are different.
 16. The LNG production system ofclaim 13, wherein the recondenser switches to perform, alternately, afirst recondensation in a recondenser that recondenses boil off gas thatis generated by the liquid natural gas, the recondensing performed byindirect heat exchange with the refrigerant fed from a secondrefrigerator, and a second recondensation wherein the boil off gas isrecondensed by indirect heat exchange with the refrigerant from thesecond refrigerator and refrigerant from a refrigerant buffer, therebyprocessing more boil off gas than the boil off gas at a time of thefirst recondensation processing.
 17. The LNG production system of claim13, wherein the recondenser is designed to recondense the boil off gasby a refrigerant under a pressure lower than an operating pressure ofthe LNG tank.
 18. The LNG production system of claim 13, wherein therecondenser is internally provided with a heat exchanger in which therefrigerant is introduced, and the boil off gas is introduced into theheat exchanger, and is cooled by the refrigerant.
 19. The LNG productionsystem of claim 18, wherein an external capacity of the heat exchangeris smaller than an internal space capacity of the recondenser, and theheat exchanger is disposed in the internal space of the recondenser. 20.A recondenser within an LNG production system that liquefies natural gasto produce liquid natural gas, and recondenses boil off gas in an LNGtank in which the liquid natural gas that is liquefied is stored, by arefrigerant, the condenser comprising: a return line that directly feedsliquid natural gas that is recondensed to the LNG tank.
 21. Therecondenser of claim 20, wherein the recondenser is designed torecondense the boil off gas by a refrigerant under a pressure lower thanan operating pressure of the LNG tank.
 22. The recondenser of claim 20,wherein the recondenser is internally provided with a heat exchanger inwhich the refrigerant is introduced, and the boil off gas is introducedinto the heat exchanger, and is cooled by the refrigerant.
 23. Therecondenser of claim 22, wherein an external capacity of the heatexchanger is smaller than an internal space capacity of the recondenser,and the heat exchanger is disposed in the internal space of therecondenser.